Yeah, I'm feeling making it some form of bramble attack, like you'd see in Golden Sun, massive thorns and maybe some blood.
The thing I keep coming back to is the size of the base. That and the visuals of the attacks. The vines/brambles are easy. It's water and fire which will prove challenging. I have a few ideas though.
EDIT: Here is a Techmarine I did years ago. I'm very proud of the human portions I did: